For consistency and also because the SafeErrorWrapperBuilder seems
to be the building pattern than the original code assumed.
New code should not use it, but I'd rather keep legacy code consistent
formally and with its own original assumptions.
In particular, it matters that SafeErrorWrapperBuilder assigns the
most relevant operation that failed. We were not doing that when we
were manually creating a new ErrWrapper.
Part of https://github.com/ooni/probe/issues/1591
8174d88bac